Biometric authentication systems are used for authenticating users of devices incorporating the authentication systems. Biometric sensing technology provides a reliable, non-intrusive way to verify individual identity for authentication purposes.
Fingerprints, like certain other biometric characteristics, are based on unalterable personal characteristics and thus are a reliable mechanism to recognize individuals. There are many potential applications for utilization of biometric and fingerprints sensors. For example, electronic fingerprint sensors may be used to provide access control in stationary applications, such as security checkpoints. Electronic fingerprint sensors may also be used to provide access control in portable applications, such as portable computers, personal data assistants (PDAs), cell phones, gaming devices, navigation devices, information appliances, data storage devices, and the like. Accordingly, some applications, in particular portable applications, may require electronic fingerprint sensing systems that are compact, highly reliable, and inexpensive.
Fingerprint sensors are sometimes referred to as “swipe” sensors or “placement” sensors depending on their principle of operation. Typically, swipe sensors capture an image that is larger than the sensing area by capturing a series of scans of the fingerprint as the user swipes their finger over the sensing area. A processing system then reconstructs the scans into a larger swipe image. Since the image is reconstructed from a series of scans, this allows the sensing array to be made small, even as small as a single scan line, while still capturing a larger area image. Placement sensors typically capture an image that corresponds to the size of the sensing area by capturing scans of the fingerprint as it is placed or otherwise held over the sensing area. Usually, placement sensors include a two dimensional sensor array that can capture a sufficient area of the fingerprint in a single scan, allowing the fingerprint image to be captured without the user having to move the finger during the image capture process.
As fingerprint sensors shrink in size, whether for the purpose of packaging them into smaller portable devices, to reduce cost, or for other reasons, accurate and usable fingerprint recognition becomes a challenging task. The fingerprint recognition system should capture a sufficient area of the fingerprint to discriminate between different users. It is possible for a swipe sensor to capture a much larger area of the fingerprint than the sensor size, allowing the fingerprint sensor to be made small while still capturing a larger area swipe fingerprint image with enough fingerprint information to easily discriminate between users. Unfortunately, some users find the process of swiping their finger over the sensor every time they want to access the system to be cumbersome.
Placement sensors provide an attractive solution for many users, since they allow the user to simply hold their finger over the sensor. However, there are several technical challenges with small placement sensors that only capture a partial fingerprint image. Because only a partial area of the fingerprint that corresponds to the size of the sensor is captured, the matching process should ideally be tailored to quickly and accurately match based on limited fingerprint information, a task for which conventional matching algorithms based on full fingerprint images are often poorly equipped. Furthermore, since the sensor is only large enough to capture a partial fingerprint image during placement, in ordinary use the user is likely to present different portions of the same fingerprint on different occasions when attempting to access the system. The recognition system should ideally be able to recognize the fingerprint without requiring the user to present the same small portion of the fingerprint every time.
To achieve this, an enrollment template is typically built up that is derived from several repeated placements of the fingerprint over the sensor. This process often still results in a low quality enrollment template, since the user is provided little guidance as to where to place the fingerprint, resulting in poor coverage of the fingerprint in the enrollment template. Furthermore, determining the geometric relationship between the separate placement views is a challenging task, particularly where multiple views are captured from non-overlapping portions of the fingerprint.